The early region of SV40 codes for two proteins of 20K and 90K. Deletion mutants that eliminate the 20K protein, temperature-sensitive mutants in the 90K proteins and double mutants have been constructed. From studies with these mutants it is clear that the 20K protein is not required for viral replication. The onset of tumor appearance in hamsters inoculated with the deletion mutants is delayed, but tumors are found with high frequency. In tissue culture, the deletions affect the frequency of transformation in resting but not in growing cells. In appropriate growth factor deficient media a requirement for the 20K or for the 90K proteins for the maintenance of transformation can be demonstrated. Depending upon the cell lines used, a direct correlation between the condition of transformation and temperature-sensitivity of the resulting cell line are found following infection with mutants that are t.s. in the 90K protein. Analyses of the pattern of integrated SV40 in many cell lines, suggest that SV40 does not integrate as a circle, but perhaps only as a tandem linear molecule. The latter suggests that SV40 may replicate by the rolling circle mode in nonpermissive infections.